This invention relates generally to engines, and more particularly to gear trains in engines for driving mechanically actuated fuel injectors.
Diesel engines are required to meet ever-reducing emission levels. Increasing the pressure to spray the fuel into the cylinders is one method of reducing emissions. Increased injection pressure requires additional torque to drive the injection system. The increased drive torque caused by high injection pressures in the unit injector fuel systems causes high-load gear impacts that generate considerable noise and occasionally mechanical failure of the gears.
For example, typically fuel pressurization in a mechanically actuated fuel injector is achieved by downward pressure on a plunger in the fuel injector. A cam operates an arm to push down on the plunger. The cam is driven by a driver gear or a driver idler gear engaged with and rotating a cam gear. While the cam is pushing against the arm to pressurize fuel tremendous force is being applied by the driver or driver idler gear against the cam gear.
When the fuel injector releases the pressurized fuel the pressure on the plunger is suddenly eliminated. With the sudden cessation of return force from the cam gear against the driver gear, the cam gear may be propelled violently forward so that the cam gear teeth can fly off the driver gear teeth and actually slam into the respective driver gear teeth in front of them. This causes considerable noise, and also contributes to gear wear.
Further, gear train strength has been increased with a change from helical gears to high contact ratio spur gears. Accordingly, the width of the gears has been increased. With every increase in injection pressure the gear loads and noise tend to increase. Accordingly it has become difficult to provide acceptable mechanical reliability with a low noise level in these gear trains with the increase in injection pressure. Larger and stronger gears, when used, cause dynamic problems of their own with their significantly increased inertia. A solution is needed to reduce the impact loads in these gear trains and otherwise address these problems.
Various techniques, including the use of torsional (viscous or rubber) dampers, absorbers, split or scissors gears, and gear backlash control techniques, have been tried. For example, U.S. Pat. No. 5,272,937 teaches an active inertia torque absorber.
These techniques have some problems. For example, the absorber and damper strategies either absorb and return the dynamic energy, or dissipate it as heat. Both of these devices have limited capacity for reducing torque. Furthermore, the added inertia of their mechanisms can increase the dynamic input. Additionally, their size can increase the weight and volume of the engine, which affects packaging and fuel economy.
Backlash control techniques with split or scissors gears can reduce the impact loads, but require a spring to force the two gears to opposite sides of the mesh. The spring in the split gear must be strong enough to be effective, yet not so forceful as to add excessive friction to the system. The split gear spring can be optimized at only one operating condition. The split gear technique requires additional axial length for packaging. Designing and producing a split gear backlash limiting system is difficult, and therefore expensive.
In a first aspect of the invention, a gear train in an engine comprises a driver, a cam, a first torque path between the driver and the cam including a first number of idlers between the driver and the cam, and a second torque path between the driver and the cam including a second number of idlers between the driver and the cam. The first number is at least zero, and the second number is greater than the first number.
In a second aspect of the invention, a method for regulating motion of a cam in an engine comprises providing a driver mechanically connected with the cam via a first torque path to provide a motive force for rotating the cam, and providing a second torque path, distinct from the first torque path, between the driver and the cam, such that rotational torque from the driver is applied to the cam at first and second respective locations on the cam. The second torque path includes a greater number of gears than the first torque path. The second torque path provides a constraint on the cam to check a sudden change in rotation speed of the cam due to a sudden change in load on the cam.